This happened to me the other day, and I was wondering if I committed a queue faux pas or not. I was in line at the grocery store, next in line and with my items on the counter (small neighbourhood grocery, no conveyer belts), when I realized that I had accidentally grabbed the wrong can (pumpkin pie filling, rather than plain canned pumpkin). Since the woman ahead of me still had quite a number of things to be scanned, I left my few items on the counter and quickly ran back to the shelves to exchange the cans, which took around 30 seconds because the store is small. When I returned, I found that a guy, who had not been in line when I'd left, had pushed my items farther away from the cashier and set his stuff in their place, taking the place before me in the queue. I was a bit surprised, since in the same situation I would have simply waited for the other person to come back (the cashier was still helping the woman in front of me) but I wonder if he was actually in the right, and I forfeited my place in line by going off to get something else. In any case, he had only a few things and I wasn't in a hurry, so I wasn't bothered (except by him touching my groceries, but that may not have been wrong either). What does everyone else think?